Ultrasonic technology is used in certain processes that are arranged for the treatment of continuous webs of material. This technology is already familiar and is suitable, for example, for the joining together of two layers of material of the nonwoven material type or other relatively thin layers of material. In the case of such joining together, also known as ultrasonic welding, a laminate is formed from the two layers of material. Such laminates are commonly encountered in the production of absorbent articles such as diapers, incontinence pads, sanitary towels and panty liners.
In addition to its use for the joining together of various materials, ultrasonic technology can also be used for other types of treatment, for example perforation, cutting, embossing of patterns or forming of material. As an example of materials that are suitable for treatment by means of ultrasonic technology, mention can be made of nonwoven material, i.e. fibrous materials for example with synthetic fibres such as polyethylene, polypropylene, polyester, nylon or the like. Mixtures of different types of fibre can also be used. Ultrasonic technology can also be used for the treatment of, for example, thermoplastic films made of polyethylene or polypropylene.
As far as concerns treatment in the form of joining together two materials intended for absorbent articles, an ultrasonic device is normally used in such a way that the materials are supplied in the form of continuous webs of material or discrete articles that are fed past or positioned in association with an ultrasonic horn belonging to the ultrasonic device and an abutment surface. This abutment surface is suitably defined by a rotating abutment roller or a plane surface which functions as an abutment. The materials are accordingly positioned in a relatively small gap that is formed between the ultrasonic horn and the abutment roller. In order to achieve the desired joining together of the two webs of material, the ultrasonic device is driven at a certain amplitude and with a certain power.
In normal applications, it is desirable to maintain a gap between the ultrasonic horn and the abutment surface that remains essentially constant. It is possible to establish, however, that a certain quantity of heat is generated during operation with the ultrasonic horn. This generation of heat normally leads to thermal expansion, inter alia in the ultrasonic horn, which in turn leads to a change in the size of the gap. A change in the size of the gap can also occur as a result of wear in the ultrasonic horn, in particular in its end part that is in fact applied to the subjacent material. This is a disadvantage in normal circumstances, since the treatment with the ultrasonic device can then lead to an unsatisfactory result. For example, an excessively small gap will lead to an excessively high level of supplied energy during treatment, which can lead to damage to the material. The opposite situation, that is to say an excessively large gap, can potentially produce an insufficient welding effect and thus poor joining together of the materials in question.
In order to solve the above-mentioned problems, the use of an ultrasonic device in which the size of the aforementioned gap can be regulated is previously disclosed. This can be done in this instance by causing the ultrasonic device to be displaced by mechanical means in relation to the abutment surface. This regulation, for the purpose of which the ultrasonic device and/or the abutment is movably arranged, can then take place dependent on various parameters which reflect the operating status of the ultrasonic device.
Patent document U.S. Pat. No. 6,190,296 shows a system of ultrasonic treatment, the purpose of which is to solve the above-mentioned problem. In accordance with this document, a sensor of the load cell type is used, with the help of which the force that acts from the ultrasonic horn and upon the abutment roller can be detected. A signal which corresponds to the measured force is fed to a control unit, which in turn is so arranged as to provide control of the ultrasonic device for regulating the size of the gap dependent on the aforementioned force. The size of the gap can thus be regulated continuously to an optimal value, so that an essentially identical level of energy is supplied to the materials during the process in question. The purpose of this in turn is to provide uniform and predictable ultrasonic treatment.
In spite of the previous disclosure of the principle of regulating the size of the gap dependent on the force with which the ultrasonic device influence the material and the abutment, it is possible to establish that the need for improvements relating to this type of regulation may arise in certain situations. For example, the prior art does not provide sufficiently uniform and dependable treatment using ultrasonic technology in respect of the joining together of two webs of material to form a laminate. If one material is narrower than the other, for example, it is difficult to obtain a uniform and strong joint between the webs of material, which in turn can be attributable to variations in dimensions, density or other characteristics of the constituent materials along the two webs of material.
Against the background of the foregoing, it is possible to establish that the need exists for arrangements and methods which utilize ultrasonic technology, and the purpose of which is to provide the more uniform, more durable and more effective sealing of laminates consisting of two or more layers of material.